1. Field of the Invention
The present invention relates to a heater element for an indirectly heated cathode and particularly to a small helically wound heating wire positioned closely to the cathode.
2. Description of the Prior Art
In cases where there is only a limited available filament power, it is possible to achieve the shortest cathode-heating time by arranging the source of heat, or the thermal center thereof, as close as possible to the electron-emitting layer of the cathode. A corresponding construction of an indirectly heated cathode has already been proposed in a copending U.S. patent application Ser. No. 418,644, filed Nov. 23, 1973. One decisive factor in bringing the thermal center of the source of heat close to the electron-emitting layer is the length of the heating element. In its usual form, the heating element consists of a coil-shaped wound heating wire, but also meander-shaped configurations have become known. In order to avoid short-circuits in the heating element, the individual windings and turns of the heating wire of the heating element must be insulated from one another.
This insulation, which substantially consists of aluminum oxide, is applied and sintered subsequent to the shaping of the heating element. During the shaping of the heating element, the mutual spacing between the individual windings and turns must be made sufficiently large to reliably avoid any contacts. The extent of this required reliability and safety determines the final spacing in addition to the electrical minimum spacing. In other words, for mechanical and technical production reasons, the longitudinal expansion of the heating element, including the insulation, is greater than electrically necessary.